1. Field of the Invention
This invention relates to centrifugal particle classifiers in general, and in particular to a forced vortex particle classifier having a uniform influx fluid distributor to enhance particle classification and throughput.
2. Background of the Invention
In the field of powder technology, classification is generally defined as the process of separating a powder into a coarse fraction and a fine fraction. The coarse comprises coarse particles having sizes equal to and larger than a cut size, whereas the fine fraction comprises fine particles having sizes equal to and less than the cut size. The cut size is equivalent to the separation point, i.e., that particular size of particles about which the powder is separated.
Numerous types of classification systems exist and have been used with varying degrees of success to separate or classify the powder into the coarse and fine fractions. Centrifugal air classifiers are among the more common types of classification systems and divide airborne powders (aerosols) by subjecting them to high centrifugal forces and opposing airflows. As is well-known, the centrifugal forces dominate the dynamics of large particles, whereas aerodynamic drag forces (Stokes forces) dominate the dynamics of the smaller particles. In a centrifugal classifier, the large particles above the design cut size are thrown outwardly by centrifugal forces and are therefore effectively separated from the small particles, which remain entrained in the classifying air.
Centrifugal classification systems can take many forms, but usually can be grouped into one of two classes: "free" vortex systems or "forced" vortex systems, depending on the particular means used to create and maintain the vortices in the classification chambers. Most free vortex classifiers use curved vanes or stators to generate the vortices, whereas most forced vortex classifiers use spinning rotors to establish and maintain the vortices.
Most forced vortex centrifugal classifiers have a rotor or classifier cage mounted for rotation within a hollow stator, so that the rotor and stator are separated by a narrow annular air gap. The rotor defines a hollow coaxial chamber that is in communication with the air gap along the periphery of the rotor and also in communication with a central opening for the egress of the classifying fluid, usually air, along with the entrained fine fraction of the powder. Typically, the classifying air is supplied to the chamber through the gap and a vortex is produced generally within the rotor by the rotation of the rotor itself. The powder to be classified is supplied to the vortex and the coarse fraction of the powder, which is forced by centrifugal action towards the stator, is removed through a coarse fraction passageway, while the fine fraction is removed with the fluid through the central axial opening in the rotor.
Depending on the particular design of the forced vortex centrifugal classifier, the powder may enter the air gap through the stator along a radius of the chamber, so that the classifying air and the powder initially enter the air gap traveling perpendicular to each other. However, other forced vortex classifiers do not require that the powder and classifying air be separate, and instead mix the powder with the classifying air before it is introduced into the classifier. Examples of forced vortex centrifugal classifiers can be found the patents issued to Nomar, U.S. Pat. No. 2,991,844; Bouru, U.S. Pat. No. 3,561,195; Lapple U.S. Pat. No. 3,720,313; Voelskow, U.S. Pat. No. 3,767,045; Erickson, U.S. Pat. No. 4,268,281; Barthelmess, U.S. Pat. Nos. 4,409,097 and 4,390,419.
The material handling capacity or throughput of a forced vortex centrifugal classifier is principally dependent on the axial length and diameter of the rotor or classifier cage, i.e., its circumferential surface or the cylindrically annular chamber in which the classification is performed. The other essential parameter of the classification process, namely the particle size limit differentiating the fine material from the coarse material (the cut size), is on the one hand determined by the diameter and rotational speed of the classifier cage and on the other hand by the external diameter of the classifying chamber and on the inflow rate of the classifying air into the classifying chamber. In both cases, the cut size is dependent on the centrifugal forces acting on the particles being classified.
While it is theoretically possible to increase the throughput of the classifier by increasing the diameter of the centrifuge cage, practical restrictions on the cage diameter exist since the centrifugal forces increase on a square law basis, thus rapidly increasing the forces acting on the centrifuge cage. Another factor limiting the size of the centrifuge cage diameter is the decrease in curvature of the classifying air path (i.e., the vortex) as the diameter of the cage increases. This curvature increase can be compensated by higher outflow speed or higher classifying air quantities, but these and the increasing resistance and frictional losses impose unacceptably high power requirements.
The axial length of the centrifuge cage is also limited due to the increasing torsional loading (wind-up) and axial deflection of the cage shaft that typically accompany an increase in cage length. However, these structural problems usually can be reduced by proper cage design and by supporting the cage shaft at both ends. However, even if these structural problems are solved, the axial length of the centrifuge cage is particularly limited by the varying airflow rates along the cage edges where the classifying air and fines pass between the blades.
Essentially, the flow rates along the jacket-like circumferential surface of the centrifuge cage are directly related to the suction within the cage. This suction is at a maximum level at the central coaxial opening (fines outlet) from the cage and decreases towards the closed end of the cage. These varying flow rates along the length of the cage lead to differences in the separation quality or selectivity of the classifier. As a result, oversize material passes into the fines in the vicinity of the fines outlet where maximum suction action occurs, while at the greatest distance from the fines outlet, undersize material will remain with the coarse material and be rejected with the coarse fraction. These selectivity disadvantages increase with the magnitude of the axial length of the centrifuge cage, and heretofore have limited the throughput of such classifiers if an acceptably narrow cut size is to be maintained.
One solution to overcome the aforementioned disadvantages resulting from the different flow rates along the axial direction of the cage has been to add an additional fines outlet to the other end of the cage, as disclosed in European Patent 67 895B1. However, this is not a complete solution, as there will still be diminished air flow at the midpoint of the cage. Further, providing such an additional fines outlet can lead to difficulties in adequately supporting both ends of the cage, increasing the chances for cage vibration and flutter.
The patent issued to Hanke, U.S. Pat. No. 4,869,786 recognizes this problem and instead solves the throughput limitation imposed by the flow variations along the cage by utilizing a multi-stage design with a plurality of centrifugal cages and chambers. More specifically, Hanke provides two stages, a pre-classification and a re-classification stage, to achieve improved separation efficiency and throughput. Unfortunately, however, Hanke's multi-stage design is large and cumbersome and requires relatively complicated apparatus.